There are widely known diecasting machines of the type that a molten metal material is injected and filled in a cavity of a mold to obtain a product. With such a diecasting machine, a metal material (for example, Al alloy, Mg alloy or the like) which has been molten in a smelting furnace is metered and lifted out by a ladle upon every shot, and the thus-ladled molten metal (molten metal material) is poured into an injection sleeve and is then injected and filled in a cavity of a mold by an advancement of an injection plunger.
An injection step which is performed by such a diecasting machine generally consists of a low-speed injection step and a high-speed injection step, and in the high-speed injection step, it is necessary to inject and fill molten metal in a mold at a high injection speed which is faster by one digit or so than the injection speed of an injection molding machine for plastics. In general, a relatively large hydraulic drive source has, therefore, been used as an injection drive source conventionally. Reflecting the use of a hydraulic drive source as an injection drive source as mentioned above, hydraulic diecasting machines in which drive sources for mold open and closure and ejection are also designed as hydraulic drive sources have been the mainstream of diecasting machines.
Hydraulic diecasting machines, however, involve the potential risk of smear with oil or the like. There is, accordingly, an increasing desire toward electrically-driven, clean diecasting machines in recent years. Concerning such electrically-driven diecasting machines, there is known, for example, the technology disclosed in JP-A-2000-84654 and JP-A-2001-1126. According to the technology disclosed in these patent publications, a diecasting machine is equipped with an electric servomotor for injection and also with an accumulator as a hydraulic drive source to be used in a pressure-raising step and a pressure-holding step. A low-speed injection step and high-speed-injection step during an injection step are performed only by the drive force of the electric servomotor for injection. The pressure-raising step is performed by combining the drive force of the electric servomotor for injection and that of the accumulator. Further, the pressure-holding step which follows the pressure-raising step is performed only by the drive force of the accumulator. As an alternative, the pressure-raising and pressure-holding steps are performed only by the drive force of the accumulator.
According to the technology disclosed in JP-A-2000-84654 and JP-A-2001-1126 referred to in the above, the electric servomotor is used as the drive source for the injection step (the low-speed injection step and high-speed injection step), and the force of the hydraulic drive source is used only for the pressure-raising and pressure-holding steps. This conventional technology, therefore, makes it possible to make the size of a hydraulic system smaller, to realize a relatively clean diecasting machine, and to readily output a large pressure upon raising the pressure. To achieve a high injection speed in the high-speed injection step, however, this conventional technology relies solely upon the power of the electric servomotor. Accordingly, there is a certain limit to the acceleration of the injection speed, and moreover, a relatively large motor is also required as the electric servomotor to assure the high injection speed.